What is this spikes in CPU usage your talking my friend ?
Your CPU @ 4.5Ghz is known to handle the bulldozer,, why you want to throw your money away on a pointless upgrade ???
It has 8 cores, but so does Intel , if you use 64 bit HT app. Which I use, DAW Sonar X1c Producer ,,, or if you use adobe Premiere or Sony Vegas , the 8 core FX bulldozer will shine in these apps. If your not using the above apps,, stop thinking about this and just chill with your 4.5GHz Sandy,, I mean theres kids dying everyday in Africa ..and here you are literally wanting to throw away hundreds of dollars for no reason.
Maybe is your COCD ,, computer obsessive compulsive disorder but your Sandy @ 4.5Ghz will handle anything you throw at it including Premiere and DAW etc....
stop looking at the cpu usage, it should be 1 percent or 0 percent ,,, listeng to winamp and surfing. Look at your task manager maybe something is taking that CPU usage,, that shouldnt be there, a virus or a stupid app.. ghmmmmmm GL